- eliminate CMake 3.5+ warnings
- removed usage of dropped "OPENCV_CAMERA_MODULES" variable
- partial support for Android NDK 11+

OPENCV_ANDROID_SDK - OpenCV Android SDK location (or OpenCV build dir)
Can be passed via ndk-build parameter or environment variable.

put the detection-based tracker back (to objdetect module), so that the Android face detection sample builds fine. The patch by Konstantin Matskevich that adds C++11 threads support has been merged in too. Also, fixed compile error in the doc builder.

Conflicts:
	modules/core/include/opencv2/core/cuda.hpp
	modules/cudacodec/src/thread.cpp
	modules/cudacodec/src/thread.hpp
	modules/superres/perf/perf_superres.cpp
	modules/superres/src/btv_l1_cuda.cpp
	modules/superres/src/optical_flow.cpp
	modules/videostab/src/global_motion.cpp
	modules/videostab/src/inpainting.cpp
	samples/cpp/stitching_detailed.cpp
	samples/cpp/videostab.cpp
	samples/gpu/stereo_multi.cpp

(#2569) simplifying code to avoid Eclipse CDT editor confusion, removing warnings and typos in comments

- Adding Android SDK/ADT r21 compatibility;
- fixing stl headers path for new ndk;
- fixing native API level for Android projects

Face detection package name replaced on org.opencv.facedetect for consistency with Google Play package.

Revision 8725 restored. Android sample FaceDetection updated according DetectionBasedTracker interface.

revision 8721 vas merged to head. Detection based tracker interface changed. cpp and android samples updated.

Warning: changes beak binary compatibility